1. Field of the Invention
The present invention relates to a power supply device that uses a permanent-magnet generator, including a control unit therefor.
2. Description of the Related Art
As an example of a conventional power supply device using a permanent-magnet generator, there is known a device that includes a permanent-magnet generator, a full-wave rectification circuit made up of diodes, and a generation-voltage control circuit made up of thyristors connected in parallel with low side diodes of the full-wave rectification circuit in such away that the anode of each thyristor is connected to an input terminal of the permanent-magnet generator and the cathode thereof to ground.
In such a generation-voltage control circuit as described above, when power needs to be supplied to an electrical load, if the electrical load includes an electric storage unit and a terminal voltage across the load is low, the thyristors are gate-turned off and AC power generated by the permanent-magnet generator is converted into DC power by the rectification circuit and supplied to the electrical load. Meanwhile, when the electrical load does not need power in such cases as the load includes an electric storage unit and its terminal voltage is high enough, the thyristors are gate-turned on and output terminals of the permanent-magnet generator are short-circuited with each other so as to control the generation voltage.
In the above-described conventional power supply device using a permanent-magnet generator, power losses arise due to voltage drop across the diodes of the full-wave rectification circuit during electricity generation, and that across the thyristors of the generation-voltage control circuit and the low side diodes of the full-wave rectification circuit during the short-circuit, which has in turn caused such problems as generation efficiency of the power supply device decreasing and its radiator increasing in size due to an increase in heat generation.
Therefore, in order to solve such problems with the conventional device described above, another power supply device has been conventionally devised as disclosed in, for example, Japanese Patent No. 2858008, hereinafter referred to as Patent Document 1. The device shown in Patent Document 1 includes MOSFETs, for low side elements of the full-wave rectification circuit, that produce less power losses than the diodes and thyristors. The device operates in full-wave rectification mode in which when an output terminal voltage from the permanent-magnet generator is positive, the MOSFETs are turned off, whereas when the output terminal voltage negative, the MOSFETs are turned on, thereby supplying power to the electrical load. Meanwhile, when the terminal voltage across the electrical load is higher than a predetermined value, the MOSFETs are turned on so that input terminals of the permanent-magnet generator are short-circuited with each other, thereby reducing power losses due to voltage drop across the diodes and thyristors.
However, in the conventional device shown in Patent Document 1, since control power for controlling the power supply device is supplied from the electrical load side (electric storage unit side), in cases such as when connection between the power supply device and the electrical load is interrupted, the control circuit for the MOSFETs is not supplied with enough power, so that the MOSFETs cannot be properly driven, causing a problem in that the permanent-magnet generator becomes unloaded, impressing an overvoltage on the power supply device.